Capacitive touch sensors are used as a user interface to electronic equipment, e.g., computers, mobile phones, personal portable media players, calculators, telephones, cash registers, gasoline pumps, etc. In some applications, opaque touch sensors provide soft key functionality. In other applications, transparent touch sensors overlay a display to allow the user to interact, via touch or proximity, with objects on the display. Such objects may be in the form of soft keys, menus, and other objects on the display. The capacitive touch sensors are activated (controls a signal indicating activation) by a change in capacitance of the capacitive touch sensor when an object, e.g., a user's finger tip, causes the capacitance thereof to change.
Today's capacitive touch sensors come in different varieties, including single-touch and multi-touch. A single-touch sensor detects and reports the position of one object in contact or proximity with the touch sensor. A multi-touch sensor detects the position of one or more objects in simultaneous contact or proximity with the touch sensor, and reports or acts upon distinct position information related to each object.
A touch sensor used in both single- and multi-touch systems may be constructed using one or more layers, each having a plurality of electrodes electrically insulated from each other. In a multi-layer embodiment, the layers may be fixed in close proximity to each other and electrically insulated from each other. In any of the one or more layer touch sensor constructions, the electrodes may form any type of coordinate system (e.g., polar, etc.). Some touch sensors may utilize an X-Y or grid-like arrangement. For example, in a two-layer construction, parallel electrodes on different layers may be arranged orthogonal to each other such that the points of overlap between electrodes on the different layers defines a grid (or other coordinate system). In an alternative, single-layer embodiment, the proximity relationship between one set of electrodes and another set of electrodes may similarly define a grid (or other coordinate system).
Measuring the self capacitance of individual electrodes within the touch sensor is one method employed by single-touch systems. For example, using an X-Y grid a touch controller iterates through each of the X-axis and Y-axis electrodes, selecting one electrode at a time and measuring its capacitance. The position of touch is determined by the proximity of (1) the X-axis electrode experiencing the most significant capacitance change and (2) the Y-axis electrode experiencing the most significant capacitance change.
Performing self capacitance measurements on all X-axis and Y-axis electrodes provides a reasonably fast system response time. However, it does not support tracking multiple simultaneous (X,Y) coordinates, as required in a multi-touch sensor system. For example, in a 16×16 electrode grid, the simultaneous touch by one object at position (1,5) and a second object at position (4,10) leads to four possible touch locations: (1,5), (1,10), (4,5), and (4,10). A self-capacitance system is able to determine that X-axis electrodes 1 and 4 have been touched and that Y-axis electrodes 5 and 10 have been touched, but it is not capable of disambiguating to determine which two of the four possible locations represent the actual touch positions.
In a multi-touch sensor, a mutual capacitance measurement may be used to detect simultaneous touches by one or more objects. In the X-Y grid touch sensor, for example, mutual capacitance may refer to the capacitive coupling between an X-axis and Y-axis electrode. One set of electrodes on the touch screen may serve as receivers and the electrodes in the other set may serve as transmitters. The driven signal on the transmitter electrode may alter the capacitive measurement taken on the receiver electrode because the two electrodes are coupled through the mutual capacitance. In this manner, the mutual capacitance measurement may not encounter the ambiguity problems associated with self capacitance, as mutual capacitance can effectively address every X-Y proximity relationship (node) on the touch sensor.
More specifically, a multi-touch controller using mutual capacitance measurement may select one electrode in a first set of electrodes to be the receiver. The controller may then measure (one by one) the mutual capacitance for each transmitter electrode in a second set of electrodes. The controller may repeat this process until each of the first set of electrodes has been selected as the receiver. The position of one or more touches may be determined by those mutual capacitance nodes experiencing the most significant capacitance change.
These advantages of mutual capacitance over self capacitance come at a cost. Specifically, mutual capacitance can degrade the time it takes the system to respond to a touch action when compared to self capacitance measurements. This degradation may occur because mutual capacitance is measured at each node, whereas self capacitance is measured at each electrode. In the 16×16 grid touch sensor, for example, a mutual capacitance measurement is taken at 256 nodes, whereas only 32 electrodes are measured for self capacitance.
As a result of this tradeoff, self capacitance measurements are typically employed in applications that do not require multi-touch capabilities, and mutual capacitance measurements are employed in applications that do require multi-touch capabilities. Even so, measuring every node for mutual capacitance can take a significant amount of time that may adversely affect the multi-touch system's response to a touch action.